This invention relates to apparatus and methods for performing reverse-cementing operations. More particularly, this invention relates to apparatus for use in performing reverse-cementing operations.
Typically, after a well for the production of oil and/or gas has been drilled, casing is lowered into and cemented in the well. Normal primary cementing of the casing string in the wellbore includes lowering the casing to a desired depth and displacing a desired volume of cement down the inner diameter of the casing. Cement is displaced downward into the casing until it exits the bottom of the casing into the annular space between the outer diameter of the casing and the wellbore apparatus.
The casing may also be cemented into a wellbore by utilizing what is known as a reverse-cementing method. The reverse-cementing method comprises displacing conventionally mixed cement into the annulus between the casing string and the annulus between an existing string, or an open hole section of the wellbore. As the cement is pumped down the annular space, drilling fluids ahead of the cement are displaced around the lower ends of the casing string and up the inner diameter of the casing string and out at the surface. The fluids ahead of the cement may also be displaced upwardly through a work string that has been run into the inner diameter of the casing string and sealed off at its lower end. Because the work string has a smaller inner diameter, fluid velocities in the work string will be higher and will more efficiently transfer the cuttings washed out of the annulus during cementing operations. To insure that a good quality cement job has been performed, a small amount of cement will be pumped into the casing and the work string. As soon as a desired amount of cement has been pumped into the annulus, the work string may be pulled out of its seal receptacle and excess cement that has entered the work string can be reverse-circulated out the lower end of the work string to the surface.
Reverse-cementing, as opposed to utilizing the conventional method, provides a number of advantages. For example, cement may be pumped until a good quality of cement is obtained at the casing shoe. Furthermore, cementing pressures are much lower than those experienced with conventional methods and cement introduced in the annulus free-falls down the annulus, producing little or no pressure on the formation. Oil and/or gas in the wellbore ahead of the cement may be bled off through the casing at the surface. Finally, when the reverse-cementing method is used, less fluid is required to be handled at the surface and cement retarders may be utilized more efficiently.
Although it is often desirable to utilize the reverse-cementing method, one disadvantage is that float shoes and float collars cannot be used since such float apparatus contains a back pressure check valve to prevent the flow of cement into the bottom of the casing string once the casing has reached its desired location. It is desirable, however, to use float apparatus for a number of reasons. Float apparatus prevents back flow of cement into the casing inner diameter after the cementing operations have been completed. Float apparatus also prevents oil and/or gas under high pressure from entering the inner diameter of the casing as the casing string is being run into the wellbore. If gas or oil under high pressure does enter the wellbore, it can often result in a well blowout. Additionally, the weight of the casing, particularly with deep wells often creates a tremendous amount of stress and strain on the equipment and on the casing. Float apparatus minimizes that stress as the casing is lowered into the wellbore. Thus, it is desirable to float apparatus when lowering a casing string into a wellbore, and it is also desirable to use reverse-cementing methods to cement the casing in place. The need therefore exists for float apparatus which will allow casing to be cemented utilizing a reverse-cementing method.